1. Field of the Invention
The present invention relates to a motor and a disk drive apparatus.
2. Description of the Related Art
A brushless motor is typically installed in a disk drive apparatus, such as an optical disk drive, to rotate a disk. In recent years, there has been a tendency toward higher rotational speeds of brushless motors in accordance with increasing speeds of disk drive apparatuses. Brushless motors having high rotational speeds experience a great change in rotation rate when drive thereof is started and when the drive thereof is ended.
While the brushless motor is rotated, action of magnetic flux between an armature and a magnet causes vibrations referred to as a cogging. An excitation frequency, which represents the number of vibrations of the cogging per unit time, depends on the rotation rate of the brushless motor. Therefore, a great change in the rotation rate of the brushless motor involves a great change in the excitation frequency. When the excitation frequency coincides with a natural frequency of a vibrator, resonance occurs, so that the vibration may become particularly great.
One conceivable method for avoiding the resonance as described above is to improve the rigidity of the vibrator and thereby increase the natural frequency of the vibrator. For example, it is conceivable to secure a rotor holder and a turntable, i.e., the vibrator, to each other to unite them together. In this case, a vibrator defined by the turntable and the rotor holder has an increased natural frequency. When the natural frequency of the vibrator is arranged to be higher than an excitation frequency during high-speed rotation thereof, occurrence of the resonance is prevented.
For example, it is conceivable to secure the turntable to the rotor holder by inserting projections defined on a lower surface of the turntable into holes defined in a rotor yoke.
This arrangement, however, has a problem in that the rotor yoke is fixed to the turntable while the turntable is fixed with respect to a rotation axis. That is, the rotor yoke is fixed to a shaft through the turntable. Therefore, the degree of coaxiality of the rotor yoke with respect to the rotation axis depends on the degree of precision with which the turntable is shaped. Therefore, when there are variations in the precision with which turntables are shaped, the degrees of coaxiality of rotor holders with respect to the rotation axis will vary. A poor degree of coaxiality of the rotor holder with respect to the rotation axis may promote the occurrence of vibrations of a rotating body including the turntable and the rotor yoke.